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'''Spontaneous Fermentation''', for the purposes of this article, refers to the inoculation of wort for fermentation with local ambient microbes. This is commonly achieved by use of open [[File:Tilquin blowoff tubes.jpeg|400px|thumb|right|Lambic fermenting at Tilquin with blowoff tubes]]cooling such as in a [[coolship]] where the wort is left exposed to the air and allowed to cool naturally over night and wild yeast and bacteria are introduced into the wort as it cools. Spontaneous fermentation is part of the traditional brewing method of process for [[Lambic]] <ref>[http://lambicandwildale.com/the-mystery-of-lambic-beer/ The Mystery of Lambic Beer. Jacques De Keersmaecker. Aug 1996. Retrieved 05/05/2015.]</ref>.

In the most romanticized view of spontaneous fermentation, the microbes which inoculate the wort in the coolship are sourced exclusively from the ambient environment outside the brewery. Scientific publications have suggested that in the case of some producers, these microbes may be resident in the brewhouse <ref name="Bokulic et al., 2012">[http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0035507/ Bokulich et al, 2012]</ref>. This is supported by the reluctance of lambic brewers to alter their facilities (remodeling, moving, painting, etc.) and the spraying of lambic on the walls of new buildings <ref>[https://www.facebook.com/permalink.php?story_fbid=888263374558973&id=110627652322553/ Cantillon Facebook post 5-February-2015]</ref> <ref>[http://www.latisimports.com/assets/uploads/2009/11/MBA_Boon_October_Article.pdf/ Modern Brewery Age Weekly 23-October-2009 Article by Peter Reid with Frank Boon, accessed 7-May-2015]</ref>. The microbes responsible for spontaneous fermentation may also be derived from the oak wooden [[Barrel|barrels ]] and/or foudres which are often used to hold the fermenting beer, especially if the barrels/foudres have not been thoroughly cleaned <ref name="Spitaels et al., 2015">[http://www.sciencedirect.com/science/article/pii/S074000201500012X/ Spitaels et al., 2015]</ref>. Many Belgian lambic producers thoroughly clean their barrels using hot water/steam, mechanical agitation (such as is seen at [[Cantillon]]), and/or burning sulfur <ref> Conversation between Dave Janssen and Steven Sonck of [[De Cam]], winter 2014</ref>; however even the most rigorous cleaning likely does not fully sterilize the barrels. In the case of lambic brewers the microbes resident in barrels are spontaneous in origin, having been derived from years to decades of use in the brewery without any exposure to pitched cultures and the barrels may serve as a concentrating mechanism for the desired cultures. The role of barrels as an inoculating vessel is unclear as some producers report achieving excellent results in barrels which are new to the brewery and microbially clean <ref name="Spontaneous Sour Hour" /> (~35 min in).We do not regard the use of well cleaned barrels but still containing native microbes from previous use to invalidate spontaneous fermentation

A spontaneous native wild-microbe fermentation may also be achieved by inoculating small amounts of wort and growing up the spontaneously inoculated microbes to check for suitability. This process, called a spontaneous starter, is common in homebrew production <ref> [http://www.themadfermentationist.com/2011/04/ambient-spontaneous-yeast-starters.html The Mad Fermentationist Spontaneous Starters, accessed 7-May-2015]</ref> and allows for screening of the microbes to remove wild cultures with aggressive off flavors and/or mold. This is not unlike the potential of used oak barrels, where well performing barrels may be kept and used to ferment subsequent batches (where the organisms residing in the barrel can exert their influence on the batch) while poorly performing barrels may be discarded and removed from the brewery. This process does differ from oak barrels in that native microbes are cultured and pitched into work, rather than the additional inoculation being a result of porous surfaces which cannot be fully sanitized. As different microbes survive and thrive in different environments, barrels or pre-screened and grown starters will probably not provide a complete profile of the microbes present in traditional spontaneous fermentation beers. However a combination of a coolship to inoculate the wort with ambient/brewhouse resident microbes combined with a form of pre-screening such as barrel re-use and/or spontaneous starters may provide the full microbiota present in traditional spontaneously fermented products. There is some debate as to whether using spontaneous starters should still be termed 'spontaneous fermentation' <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1225800400781505/ MTF Facebook thread about naming]</ref>. Each brewer will have to decide for him or herself what terms to use. We recommend transparency and clarity in descriptions and process to avoid potential confusion. In a stricter sense, beers using active inoculation with spontaneous starters might better be described with terms like 'wild', 'fermented with native microbes', or a description of the spontaneous starter process rather than entirely spontaneous beers. Especially if the full wort volume was not ambiently cooled and the starter was allowed to grow for some time before mixing it in. Beers receiving additions of isolated cultures or bottle dregs are not treated as spontaneous and are discussed under [[Mixed Fermentation|mixed-culture fermentation]] and [[Commercial_Sour_Beer_Inoculation|commercial sour beer inoculation]].